1. Field of the Invention
This invention relates to the preparation of polyisocyanates and is more particularly concerned with improvements in the preparation of polymethylene polyphenyl polyisocyanates by phosgenation of a mixture of the corresponding polyamines.
2. Description of the Prior Art
This invention is based on the finding that the introduction of very minor amounts of water into the reaction mixture at a certain juncture in the manufacture of an organic polyisocyanate can produce dramatic improvements in certain properties of the resulting polyisocyanate. Previous attempts have been made to modify the properties of organic polyisocyanates by treating the isolated polyisocyanate with water. Illustratively, U.S. Pat. No. 3,526,652 describes the treatment of polymethylene polyphenyl polyisocyanates with from 0.1 to 0.5 percent by weight of water at elevated temperatures. The polyisocyanate so modified is said to give rise to polyurethane foams, by reaction with a polyol, which have increased flame retardant properties.
U.S. Pat. No. 3,179,680 describes the treatment of certain aromatic polyisocyanates with minor amounts of water in order to reduce the hydrolyzable chloride content. U.S. Pat. No. 3,636,030 describes treating the still residues, remaining after distillation of aromatic isocyanates, with aqueous hydrochloric or hydrobromic acid to achieve partial hydrolysis of various components therein and then phosgenating the product to obtain additional aromatic diisocyanate.
U.S. Pat. No. 4,221,877 describes heating methylenebis(phenyl isocyanate) with water in controlled amounts to form a polyisocyanate containing biurets. The resulting isocyanate is said to give flexible polyurethane foams, when reacted with the appropriate polyol, which foams have improved physical properties as compared with the corresponding foams made from the untreated diisocyanate.
U.S. Pat. No. 3,641,094 teaches the phosgenation of primary amines in the presence of aqueous alkali to obtain the corresponding isocyanates rapidly and with utilization of relatively small proportions of phosgene to amine.
We have found that, by treating polymethylene polyphenyl polyisocyanate and related polymeric isocyanates at a specific stage prior to isolation from the phosgenation reaction mixture employed in their preparation, it is possible to produce highly significant and advantageous changes in the properties of the polyisocyanates in question. One important improvement is a significant reduction in formation of dark colored material which generally occurs during isolation of the polyisocyanate from its reaction mixture. Equally important is the production of a product which has lower viscosity and contains significantly less higher molecular weight components than is normally encountered. All these improvements are of significant commerical importance.